The present invention relates generally to dewatering methods and devices, and, in particular, to a method and device having removable filter elements for dewatering most liquid/solid solutions and slurries.
The handling of hazardous wastes, such as radioactive wastes, generated by various industrial, medical, and electrical power generating activities is both a complex and troublesome undertaking considering the potentially devastating effects of exposure to such wastes. To limit the exposure of employees who process and handle this waste is of particular importance. It is also important to process these wastes so that they remain stable during their transport and disposal. For example, radioactive wastes need processing until the radionuclides present decay to nominal levels. Accordingly, various techniques and devices have been developed to effectively process wastes.
In the case of certain radioactive wastes, such as ion exchange media, slurry wastes and sludges, one processing technique is to dewater the slurries and sludges. In order to be acceptable for disposal, radioactive wastes must be dried to less than one-half percent water by volume of waste. It is known to dewater most liquid/solid solutions and slurries by introducing these slurries to containers having a variety of fixed filter elements such as filter sheets attached to the container floor and walls, and multiple filter elements secured into fixed piping headers or racks. These dewatering devices can typically be cleaned of filter cakes that form on the filter elements during the dewatering process by briefly reversing the direction of flow, or xe2x80x9cback blowing,xe2x80x9d so that a partially cleaned or regenerated filter surface can restore the dewatering rates.
Previous dewatering devices took days or weeks to transfer and fully dewater waste slurries. Very fine or colloidal solids removed by powdered ion exchange resins may take 24 to 144 hours to transfer and dewater. Furthermore, typical filter backwash or backflush operations utilize such large quantities of air or liquids that the dewatered solids are agitated, partially refluidized, liquefied and remixed with the balance of the container contents. Even when the dewatering operations continue, there is still a substantial filter cake on the surface of the filter and the fines have been remixed with the rest of the container contents.
Therefore, there exists a need for a dewatering method and device that will more effectively and quickly transfer and dewater slurries, such as ion exchange resin slurries.
According to its major aspects and briefly stated, the present invention is generally a device and method for dewatering liquid/solid solutions and slurries. The dewatering device includes a removable filter rack and a lifting means. In particular, the device includes a container, a filter rack assembly, a filter rack lifting means, a dewatering or vacuum pump, and an air or water source for backpulsing or backflushing. These components cooperate to transfer and dewater waste slurries quickly and effectively.
The dewatering method includes the following steps: 1) lowering a filter rack to the bottom of a container; 2) slurrying wastes into the container; 3) vacuum pumping water from the slurried wastes; 4) stopping the dewatering operations when dewatering efficiency decreases from the initial rate and backpulsing the filter elements of the filter rack; 5) raising the filter rack off the bottom of the container and then optionally backpulsing the filter elements again. This process cycle of lowering filter rack, slurrying wastes, vacuum pumping, backpulsing/backflushing, raising the filter rack, and backpulsing again, is continued until the container is full of dewatered solids. Once the container is full of solids, the filter rack is removed for use in another container. Alternatively, the filter rack is left in the container and rests on top of the dewatered solids.
An alternative method of the present invention includes the following steps: 1) affixing the filter rack near the top of the container; 2) slurrying wastes into the container; 3) vacuum pumping water from the slurried wastes; 4) backpulsing the filter elements of the filter rack; 5) allowing the dewatered solids to slide down off the filter elements and settle by gravity to the bottom of the container; and 6) optionally backpulsing the filter elements. This process of slurrying wastes, vacuum pumping, backpulsing, allowing solids to settle, and backpulsing is continued until the container is full of dewatered solids. Once the container is full of solids, the filter rack is removed. Alternatively, the filter rack is left in the containers and rests on top of the dewatered solids.
A feature of the present invention is the use of backpulsing during the dewatering cycles. The backpulsing of a small volume and low pressure of air or water serves to break the filter free of the dewatered filter cake on the surface of the filter. Backpulsing at this small volume and low pressure does not mix or stir solids. Backpulsing also serves to optimize the dewatering efficiency. When the dewatering rate slows, dewatering is momentarily stopped and the filter rack is backpulsed. Thereafter, dewatering proceeds at a more efficient rate.
Another feature of the present invention is the use of the combined backpulsing and raising of the filter rack. The feature serves to effectively clean the filter elements while leaving all previously dewatered materials in a solids cake. As stated above, the backpulsing has such a small volume and pressure that no solids are mixed or stirred. Consequently, the dewatered solids remain in tact and also serve to scrape the surface of the filter elements as the latter are being lifted.
Yet another feature of the present invention is the use of the removable filter rack. Typical dewatering filters are fixed and may only be appropriate for one-time use. The filter rack of the present invention has the ability of being fully cleaned and reused. By moving the filter rack up during the dewatering cycles, the solids cakes are left at the bottom of the container and actually help to scrape the finer particles from the filter elements.
Still another important feature of the present invention is the use of the lifting mechanism. The lifting mechanism not only helps to fully clean the filter rack by lifting it, but also enables the filter rack to produce highly dewatered solids that completely fill the container with only one set of filter elements. By lifting and then relowering the rack during the dewatering cycles, the container eventually becomes full of the solids cakes that are dislodged from the filter elements.
Yet another important feature of the present invention is the arrangement of the filter elements on the filter rack. Because of particular symmetry and spacing, the filter element arrangement is designed so that the slurries are dewatered more effectively and evenly. Further, the orientation of the filter elements helps to more effectively clean the filter elements. As the rack is lifted, the vertically oriented filter elements are scraped by the solids cakes that are left behind.